This application claims priority of Japan patent Application No. 11(99)369022, filed on Dec. 27, 1999.
This invention relates to an optical waveform shaper, and more particularly relates to an optical waveform shaper capable of controlling a constant peak value of a high-speed (e.g. a speed over 10 Gbps) optical pulse.
In optical fiber communication, signals are transmitted according to whether an optical pulse exists. A peak of each optical pulse that is constant at the input stage becomes irregular due to various reasons. This phenomenon is called a pattern effect. Examples of such waveforms are shown in FIGS. 10(a) and 10(b). FIG. 10(a) shows an optical pulse train in which pulse peak values are constant. FIG. 10(b) shows an optical pulse train in which pulse peak values are irregular due to the pattern effect. Such a pattern effect results from the synergistic action of a self phase modulation effect, an interaction between adjacent optical pulses, a cross modulation effect between adjacent wavelengths and chromatic dispersion of an optical transmission fiber. Also, the pattern effect is apt to occur when the following components are used; a narrow band optical filter in which the transmission center wavelength is out of position or an optical modulator in which the modulation band is not extended enough to cover the low band. The pattern effect causes the deterioration of an eye aperture and also deteriorates a bit error rate.
Such pattern effects can be removed by applying an optical limiter for regulating a peak part of each optical pulse to be constant. The following structures are known as such types of optical limiters.
Japanese Laid-Open Patent Publication No. Heisei 10(1998)-213821 describes a waveguide type optical limiter that obtains an optical limiting function by utilizing such characteristics that, in two parallel waveguides formed on the same substrate, the coupling coefficient between the two waveguides varies according to the optical intensity applied to either one of the waveguides.
Japanese Laid-Open Patent Publication No. Heisei 9(1997)-244075 describes a configuration for obtaining the optical limiting function by appropriately selecting an absorption factor of an optical absorber. Japanese Laid-Open Patent Publication No. Heisei 9(1997)-244074 describes a configuration in which an optical amplifier is disposed before or behind the optical limiter element mentioned in the above Japanese Laid-Open Patent Publication No. Heisei 9(1997)-244075. Japanese Laid-Open Patent Publication No. Heisei 9(1997)-244073 describes a configuration in which the optical limiter mentioned in the above Japanese Laid-Open Patent Publication No. Heisei 9(1997)-244075 and the optical limiter mentioned in the above Japanese Laid-Open Patent Publication No. Heisei 9(1997)-244074 are multistage-connected.
Japanese Laid-Open Patent Publication No. Heisei 8(1996)-160367 describes a configuration in which a saturable absorber is disposed on one branched optical line in a Mach-Zehnder interferometer system which is set so that light are combined in opposite phase at a combining part. In this configuration, when intensive light enters the Mach-Zehnder interferometer system, the transmission rate of the saturable absorber increases and consequently the amount to be combined in opposite phase also increases. As a result, the power level of the output light can be limited.
U.S. Pat. No. 5,446,573 describes an all-optical waveform regenerator utilizing a nonlinear ring resonator including a light source, a multi-sectional laser for generating a clock frequency and a plurality of phase modulators.
U.S. Pat. No. 5,798,852 describes a configuration of a device for shaping a waveform of an optical signal by inputting into an all-optical gate, simultaneous with an input optical signal, the optical pulse train modulated by the same frequency with a clock frequency of the input optical signal.
U.S. Pat. Nos. 5,594,583 and 5,513,030 describe a waveform shaper for modulating the intensity of an input optical signal by applying into an electroabsorption optical modulator an electric signal modulated by the same frequency with a clock frequency of the input optical signal.
U.S. Pat. No. 5,828,478 describes a configuration for shaping a waveform of an optical signal by utilizing an intensity modulator and a phase modulator disposed in a transmission line and a nonlinear optical effect accumulated in long haul transmission. In this configuration, the transmission system as a whole has the waveform shaping function, and in that respect this configuration differs a lot from the others.
In addition to the above-mentioned patents, there are the following papers. A. Hirano, et al., xe2x80x9cAll-optical limiter circuit based on four-wave mixing in optical fibers,xe2x80x9d Electron Lett., 34, pp. 1410-1411, 1998 describes an optical limiter using four-wave mixing in optical fibers, and Y. Wang, J., xe2x80x9cNonlinear Optical Limiter and Digital Optical Switch by Cascaded Nonlinear Couplers: Analysis,xe2x80x9d Lightwave Technol., 17, pp. 292-297, 1999. 12. 19. describes an optical limiter using cascade connection of nonlinear couplers.
However, the aforesaid configurations have problems. That is, the configurations described in the Japanese Laid-Open Patent Publication No. Heisei 10(1998)-213821 and the paper by Wang, J. are inefficient since it is necessary to greatly change the propagation constant of a waveguide by the configurations need high input power. Each configuration described in the above-mentioned Japanese Laid-Open Patent Publication Nos. Heisei 9(1997)-244073, Heisei 9(1997)-244074 and Heisei 9(1997)-244075 requires a complicated optical system in order to remove the polarization dependency. The configuration described in the Japanese Laid-Open Patent Publication No. Heisei 8(1996)-160367 is inconvenient to use since its operation wavelength range is limited due to the wavelength dependency of the interferometer system.
The configurations described in the U.S. Pat. Nos. 5,446,573 and 5,798,852 are not only complicated but also require a lot of optical components. In the configurations described in the U.S. Pat. Nos. 5,594,583 and 5,513,030, the optical modulator merely modulates the intensity of the signal light uniformly, and so the limiting function can be hardly expected. The configuration described in the U.S. Pat. No. 5,828,478 requires being large-scale since the waveform shaping is performed by using the characteristics of the whole transmission system.
The configuration described in the paper by A. Hirano is inefficient since the four-wave mixing is utilized. Also, its configuration becomes complicated because it is necessary to control the polarization state of the light to be used.
As stated above, there is no such optical limiter in the conventional optical limiters or waveform shapers that can efficiently control pulse peak values of an optical pulse train to be constant and still has a simple configuration. Also, the high-speed responsibility is insufficient in the conventional art.
It is therefore an object of the present invention to provide an optical waveform shaper of a simple configuration for regulating a peak value of an optical pulse to be constant.
Another object of the invention is to provide an optical waveform shaper for controlling a peak value of an optical pulse, as fast as 10 Gbps or more, to be constant.
An optical waveform shaper according to the invention is composed of a probe light source for generating a probe light having constant amplitude, a signal copier being applied with input signal light of a wavelength xcexs and the probe light output from the probe light source and for copying the signal of the input signal light onto the probe light, and an extinction ratio improver for improving the extinction ratio of output light from the signal copier. The signal copier copies the signal of the input signal light onto the probe light having the constant amplitude, and the extinction ratio improver improves the extinction ratio of the signal. This configuration is sufficient for the signal copier to shape the waveform of the probe light to the extent capable while still maintaining the signal of the input signal light, and so the amplitude of the probe light after the waveform shaping still keeps the constant value. The signal quality is also improved because of the improvement of the extinction ratio by the extinction ratio improver, and so an optical waveform shaper having satisfactory characteristics as a whole can be realized.
The signal copier is composed of, for instance, a waveform copier for copying a signal waveform of the input signal light onto the probe light with cross saturable absorbing characteristics. The waveform copier is composed of, for instance, an electroabsorption optical modulator. With this configuration, high-speed input signal light as fast as 10 Gbps or more can be realized.
The extinction ratio improver is preferably composed of a saturable absorber. The saturable absorber is composed of, for example, an electroabsorption optical modulator being applied with a predetermined DC bias. With this configuration, an extinction ratio of high-speed signal light as fast as 10 Gbps or more can also be realized.
When the optical waveform shaper has an optical circulator for introducing the input signal light into the signal copier as well as applying the probe light from the signal copier to the extinction ratio improver, the configuration becomes even simpler.
Preferably, the probe light source is composed of a pulse light source for generating an optical pulse having the same frequency as a clock frequency of the input signal light. With this configuration, the waveform quality of the output light pulse is also improved.
The probe light source is further composed of a phase synchronizer for synchronizing the phase of the optical pulse from the pulse light source with the input signal light. The phase synchronizer is, for instance, a phase adjuster for adjusting the pulse phase of the pulse light source according to the output light of the extinction ratio improver. With this configuration, the signal of the input signal light is converted into the probe light without difficulty even if the input signal light has jitter more than a certain amount.